What is the best software to use for music manipulation/remixing?

Question by hibikirush: What is the best software to use for music manipulation/remixing?
I’ve had an interest in remixing music/making mash-ups/etc for a while but have no idea where to start. I don’t want to do it seriously but just want to play around with it as a hobby. What is the best software (free or paid) to use for this kind of thing? And are there any tutorials online?